Mazda is offering a sneak peek at its new crossover, the CX-5, which will make its debut at next month’s Frankfurt Motor Show. While the sculpted styling reveals the practical application of the Japanese maker’s new Kodo design language, what really matters is hidden under the hood.

The SkyActiv powertrain is designed to give the CX-5 and other Mazda products a significant bump in both performance and, perhaps more importantly, in fuel economy. Like its competitors, Mazda is struggling to meet consumer demand for better mileage — a push that will come to shove now that the Obama administration has approved plans to boost the corporate average fuel economy, or CAFE, standard to 54.5 mpg by 2025.

That’s a modest retreat from earlier proposals that would have stretched as high as 62 mpg, but even though industry leaders grudgingly endorsed the new rules, they caution it will be a costly and difficult challenge that may require a significant number of technological breakthroughs to achieve.

The internal combustion engine won’t go away, not that soon, anyway, so gasoline engines will apply even more advanced forms of direct injection, more aggressive turbocharging and possibly the use of lasers to replace conventional spark plugs.

New and even cleaner diesels will almost certainly migrate from Europe to the U.S. Chevrolet hopes to get more than 50 mpg on the highway with the diesel-powered Cruze sedan it announced last month.

Automatic transmissions will continue to add more gears, like the 9-speed Chrysler is developing, while more cars will switch to “manumatic,” or clutchless manuals.

Downsizing will be essential. Ford is working on a 1.0-liter, 3-cylinder version of its EcoBoost, while pint-sized V-6s could deliver V-8 performance in its big F-Series pickups.

A number of experts believe that the new CAFE rules may force the industry to migrate more and more towards alternatives to the internal combustion engine.

One possibility is OPOC, being developed by Detroit-based EcoMotors International.

Short for “opposed piston/opposed cylinder,” the engine can run on a wide range of fuels and proponents claim it can yield markedly improved fuel economy from a package a fraction of the size of a conventional engine. The claims have resonated with Microsoft founder Bill Gates, who has pumped plenty of cash into the start-up, while truck-maker Navistar International is looking at ways to use the technology in its big trucks.

There are a number of other alternative engines being touted as the answer to the CAFE challenge, but they all face a major hurdle: automakers have been loath to make such a major change since they’ve invested so heavily in the development and manufacture of conventional gas and diesel powertrains.

“There’s still a lot of juice left to get out of the orange,” says AutoPacific analyst Dave Sullivan, of the current internal combustion engine. “But being able to spread the cost around,” as makers consider alternatives, like OPOC, they could use around the world may just open the door to alternative powertrains.

It almost certainly will result in increased use of battery-based technologies. Few expect the electric vehicle to take over anytime soon — not without major battery breakthroughs. But expect to see more and more gas- or diesel-electric models in various flavors; mild, full and plug-in.

By 2025, planners believe, almost every car will offer some basic battery technology, starting with so-called Stop/Start systems. Come to a light or wait in a fast-food line and the engine will automatically shut off, instantly restarting when you lift your foot off the brake.

Perhaps the most radical powertrain solution would migrate the nation’s automotive fleet to hydrogen power. Fuel cell vehicles have the added advantage of producing nothing but water vapor as their exhaust.

Even the most advanced engines will require other significant engineering changes. Industry designers already place a premium on aerodynamics, wind-cheating designs capable of boosting a vehicle’s fuel economy by 5, 10, even 20 percent when well executed. A number of new models, including the Cruze Eco, use shutters to block off the grille and reduce drag when engines are running cool. Don’t be surprised to see side-view mirrors go away, replaced by tiny cameras and in-car monitors.

Tomorrow’s tires will continue to reduce rolling resistance, using new rubber — or synthetic compounds — and advanced tread designs.

Like much of America, today’s cars have gained a lot of weight over the years, in part due to regulatory demands, but also because of consumer desires for more comfortable seats, louder stereos and in-car navigation systems.

Toyota has paired with JBL to develop the Green Edge audio system, which cuts in half the weight of the sound system on the new Prius v and next-generation Camry models.

BMW will make extensive use of strong yet ultra-light carbon fiber on its new i3 and i8 models, the first two entries into a new battery-car sub-brand. The German carmaker has invested heavily in efforts to transform the costly composite from a niche to mainstream material and could show off the results on the small Mini Rocketman it is developing.

There are plenty of other technologies that the new CAFE rules — and even stiffer European mileage requirements — will demand, like systems that can capture and re-use energy normally lost as heat.

One thing is certain though: The cars of 2025 will be so technically sophisticated that they’ll make the average automobile of 2011 look about as primitive as the Model T seems to us today.